In a startup, that's all about customer development. Steve Blank has a book and a lot to say on the topic. http://www.amazon.com/gp/product/0976470705/ref=as_li_ss_tl?...
If so, how should it flow?
UX is non-trivial you will have to spend a lot of time learning about it. I always recommend Don't Make Me Think as a starting point. http://www.amazon.com/gp/product/0321344758/ref=as_li_ss_tl?...
How important is...
There are questions that can be answered by understanding UX patterns and anti-patterns. There are also questions that require A/B testing. When in doubt, test.
However, UX can also suffer from premature optimization. Just like in writing code where you wouldn't use a big array when you need a map, don't ignore UX, but don't try to answer details until after it is working or you could find yourself bike shedding instead of shipping.
Don't make me Think by Steve Krug http://www.amazon.com/Dont-Make-Me-Think-Usability/dp/032134...
Non-Designer's Design Book http://www.amazon.com/Non-Designers-Design-Book-Typographic-...
Universal Principles of Design http://www.amazon.com/Universal-Principles-Design-Revised-Up...
Also make sure you go through design's everyday on Behance.net, they have some really amazing designers and design.
http://www.amazon.com/Influence-Psychology-Persuasion-Busine...
The 22 Immutable Laws of Marketing - Violate Them at Your Own Risk!
http://www.amazon.com/22-Immutable-Laws-Marketing-Violate/dp...
Don't Make Me Think: A Common Sense Approach to We Usability, by Steve Krug:
http://www.amazon.com/Dont-Make-Me-Think-Usability/dp/032134...
Information Architecture for the World Wide Web: Designing Large Scale Web Sites, by Paul Rosenfeld and Peter Morville:
http://www.amazon.com/Information-Architecture-World-Wide-We...
Why We Buy: The Science of Shopping, by Paco Underhill
http://www.amazon.com/Why-We-Buy-Shopping-Updated-Internet/d...
And at least one Jakob Nielsen Usability book.
- Don't Make Me Think by Krug: http://www.amazon.com/Dont-Make-Me-Think-Usability/dp/032134...
- Apartment Therapy by Gillingham-Ryan http://www.amazon.com/Apartment-Therapy-Eight-Step-Home-Cure...
- Presentation Zen by Gar http://www.amazon.com/Presentation-Zen-Simple-Design-Deliver...
- Belisarius Saga by Flint and Drake http://baencd.thefifthimperium.com/13-TheBalticWarCD/TheBalt...
- Wise Man's Fear by Rothfuss http://www.amazon.com/Wise-Mans-Fear-Kingkiller-Chronicles/d...
- Wizard of Oz by Baum http://www.amazon.com/Wizard-Oz-Puffin-Classics/dp/014132102...